Update proposal for custom attributes schema

design-documents/graph-ql/coverage/custom-attributes/custom-attributes-container.md

@@ -8,13 +8,42 @@ One workaround for "getting all fields" is based on schema introspection, it all
 
 # Proposed solution
 
-To account for dynamic nature of EAV attributes and the need of "getting all fields" in product search queries, `custom_attributes: [CustomAttribute]!` container will be introduced. 
+To account for dynamic nature of EAV attributes and the need of "getting all fields" in product search queries, we can introduce `custom_attributes: [CustomAttribute]!` container (recommended approach). 
 
 ```graphql
 type CustomAttribute {
     code: String!
+    values: [String]! # We want to account fo attributes that have single (text, dropdown) and multiple values (checkbox, multiselect)
+}
+```
+
+We could also make value complex type to be able add more fields in the future, but this doesn't seem necessary at this point.
+
+```graphql
+type CustomAttribute {
+    code: String!
+    values: [CustomAttributeValue]!
+}
+
+type CustomAttributeValue {
+    value: String!
+}
+```
+
+Alternative approach would be is to introduce an interface `custom_attributes: [CustomAttributeInterface]!`, but it seems more complicated.
+
+```graphql
+type CustomAttributeInterface {
+    code: String!
+}
+
+type SingleValueCustomAttribute extends CustomAttributeInterface {
     value: String!
 }
+
+type MultipleValuesCustomAttribute extends CustomAttributeInterface {
+    values: [String]!
+}
 ```
 
 Here `value` is JSON-encoded value of the custom attribute.